Such a steel ball valve, adapted to uses under high pressures and at high temperatures, is described in French Patent No. 84 08177/2 564 558 or in the corresponding European Patent No. 0 165 186, in the name of the applicant. Through the use of metal membranes, or equivalent means such as bellows sections, and of equilibration means, this type of valve makes it possible to release the seat on the "upstream" side when the valve is opened, so that said seat is no longer pressed against the ball. In this case, the ball can rotate easily, without friction, and the valve can easily be adjusted to the open position with no risk of jamming.
Moreover, in closed position, the difference in pressure between the upstream and downstream parts of the valve and exerted on the metal membrane causes the seat to be pressed against the ball and thus ensures the seal of the valve in the upstreamdownstream sense, the seat being endowed with an annular sealing lip at its end closest to the ball.